Recently, a device which treats image information as digital and compresses and encodes an image by using redundancy unique to the image information for the purpose of highly efficient information transmission and accumulation and by employing an encoding scheme where compression is performed through orthogonal transform such as discrete cosine transform and motion compensation. As the encoding scheme, there are, for example, MPEG (Moving Picture Experts Group), and the like.
Particularly, MPEG2 (ISO/IEC 13818-2) is defined as a general-purpose image encoding scheme and is a standard which covers both of an interlaced scanning image and an progressive scanning image, a standard-resolution image, and a high-definition image. For example, currently, the MPEG2 has been widely used in a wide range of applications for professional purposes and consumer purposes. By using the MPEG2 compression scheme, in the case of a standard-resolution interlaced scanning image having, for example, 720×480 pixels, a code amount (bit rate) of 4 to 8 Mbps can be allocated. In addition, by using the MPEG2 compression scheme, in the cased of a high-resolution interlaced scanning image having, for example, 1920×1088 pixels, a code amount (bit rate) of 18 to 22 Mbps can be allocated. Accordingly, it is possible to realize a high compression ratio and a good image quality.
The MPEG2 is mainly intended for high image quality encoding suitable for broadcasting, but it does not cope with an encoding scheme for a higher code amount (bit rate) than that of MPEG 1, that is, for a higher compression ratio. With the spread of mobile terminals, demands for such encoding scheme are considered to be increased in the figure, and thus, an MPEG4 encoding scheme was standardized for coping with this. With respect to the image encoding scheme, the standard was approved as International Standard ISO/IEC 14496-2 in December, 1998.
In addition, recent years, originally for the purpose of image encoding for TV conferencing, standardization of H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has proceeded. It is known that, in comparison to the encoding scheme such as MPEG2 or MPEG4 in the related art, the H.26L requires a large calculation amount by the encoding and the decoding, but a higher encoding efficiency is realized. In addition, at present, as a part of activities of MPEG4, standardization which is based on the H.26L and incorporates a function which is not supported by the H.26L so as to realize a higher encoding efficiency has proceeded as Joint Model of Enhanced-Compression Video Coding.
As a schedule of the standardization, in March, 2003, International standard was made on the basis is of the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter, referred to as AVC).
Furthermore, as an extension of the H.264/AVC, the standardization of ERExt (Fidelity Range Extension) including encoding tools necessary for business such as RGB, 4:2:2, or 4:4:4, 8×8 DCT or quantization matrices defined by MPEG-2 was completed in February, 2005. Accordingly, by using the H.264/AVC, an encoding scheme capable of representing even film noise included in a movie well, so that it is used in a wide range of applications such as Blu-Ray Disc (trade mark) in progress.
However, in recent years, demands for much higher compression ratio encoding desired to compress an image of about 4000×2000 pixels which is four times the resolution of a high-definition image or desired to deliver a high-definition image in an environment of a limited transmission capacity such as the internet have been increased. Therefore, in the aforementioned VCEG of the ITU-T, studies on the improvement of the encoding efficiency have continued to be made.
Therefore, at present, for the purpose of improving the encoding efficiency further than the AVC, JCTVC (Joint Collaboration Team-Video Coding) which is a joint standardization organization of the ITU-T and ISO/IEC has proceeds standardization of an encoding scheme called HEVC (High Efficiency Video Coding). With respect to the HEVC standard, a committee draft, as a first draft of specifications was issued in February, 2012 (for example, refer to Non-Patent Document 1).
In the HEVC standard disclosed in Non-Patent Document 1, a scheme which is referred to as “intra Transform Skipping” is employed (for example, refer to Non-Patent Document 2).
Namely, first, a flag in a PPS (picture parameter set) as to whether to apply transform skip (sometimes, referred as orthogonal transform skip) to the picture is transmitted. The orthogonal transform skip denotes that an orthogonal transform process/inverse orthogonal transform process is skipped (omitted).
When the value is 1, the orthogonal transform skip can be applied to 4×4 orthogonal transform blocks (TU). For each block, a flag on on/off of the orthogonal transform skip is transmitted. With respect to the block to which the orthogonal transform skip is applied, there is no change into processes such as entropy encoding, quantization, and loop filtering.
In other words, for the block to which the orthogonal transform skip is applied, the orthogonal transform/inverse orthogonal transform is not performed, but the encoding or the decoding is performed. For example, a before-orthogonal-transform difference value is quantized and lossless-encoded. In the case of decoding the encoded data obtained in this manner, as a matter of course, the inverse orthogonal transform process is not required.
The orthogonal transform skip is particularly effective in improving an image quality of CG images or screen content such as captions.